(a) Field of the Invention
This invention relates new compositions comprising unacylalted ghrelin and their therapeutical uses thereof.
(b) Description of Prior Art
Ghrelin is a recently discovered gastric hormone of 28 amino acids showing a unique structure with an n-octanoyl ester at its third serine residue (Kojima M et al. Nature 1999; 402(6762):656-660). Though many synthetic peptidyl and nonpeptidyl growth hormone (GH) secretagogues (GHS) were identified as ligands of GHS-R, ghrelin is shown to be a physiological ligand for the GHS-R. Ghrelin powerfully stimulates GH secretion through its interaction with GHS-R both in animals and in humans (Ukkola, O et al., 2002 Ann. Med. 34:102-108). The GH-releasing activity of ghrelin is mediated by activation of GHS-R at the pituitary and, mainly, at the hypothalamic level (Kojima M et al. Nature 1999; 402(6762):656-660) likely by enhancing the activity of growth hormone releasing hormone (GHRH)-secreting neurons and, concomitantly, acting as a functional somatostatin (SS) antagonist (Ghigo E et al. Eur J Endocrinol 1997; 136(5):445-460). Other mechanisms have been postulated recently as well (Ahnfelt-Ronne I et al. Endocrine 2001; 14(1):133-135). The interplay among various factors leading to GH secretion is depicted in FIG. 1.
The GHS-R and its subtypes are not restricted to the hypothalamus-pituitary unit but are present also in other central and peripheral tissues (Papotti M et al. J Clin Endocrinol Metab 2000; 85(10):3803-3807) and the physiological actions of ghrelin, as well as those of synthetic GHS are not restricted to GH secretion. In fact, ghrelin stimulates lactotroph and corticotroph hormone secretion, has orexigenic and cardiovascular actions, shows anti proliferative effects on thyroid and breast tumors and regulates gastric motility and acid secretion through vagal mediation (Ukkola, O et al., 2002, Ann. Med. 34:102-108).
In humans, fasting leads to elevated serum GH concentrations. Traditionally, changes in hypothalamic GHRH and somatostatin have been considered as the main mechanisms, which induce elevations in GH secretion during fasting. As ghrelin administration in man also stimulates GH release, and serum ghrelin concentrations are elevated during fasting, increased ghrelin actions might be another mechanism whereby fasting results in the stimulation of GH release.
Although ghrelin is likely to regulate pituitary GH secretion in interplay with GHRH and SS, GHS receptors have also been identified on hypothalamic neurons and in the brainstem (Nakazato M et al. Nature 2001; 409(6817):194-198). Apart from potential paracrine effects, ghrelin may thus offer an endocrine link between the stomach, hypothalamus and pituitary, suggesting an involvement in the regulation of energy balance. Tschop et al. have shown that daily peripheral administration of ghrelin in mice and rats caused weight gain by reducing fat utilization (Tschop M et al. Nature 2000; 19; 407(6806):908-913). Intracerebroventricular administration of ghrelin generated a dosedependent increase in food intake and body weight. Rat serum ghrelin concentrations increased by fasting and decreased by re-feeding or oral glucose administration, but not by water ingestion. Apparently ghrelin, in addition to its role in regulating GH secretion, signals the hypothalamus when an increase in metabolic efficiency is necessary (Tschop M et al. Nature 2000; 19; 407(6806):908-913; Muller A F et al. Clin Endocrinol (Oxf) 2001; 55(4):461-467).
Studies by Kojima and others have shown that unacylated ghrelin (UAG) has no affinity to the known GHS-R (GHS-R1a receptor), which is responsible for GH release from the pituitary gland (Kojima M et al. Nature 1999; 402(6762):656-660). This was confirmed later by Bednarek M A et al (Bednarek M A et al, J. Med Chem. 2000, 43:4370-4376), who showed that unacylated ghrelin could not be a physiological ligand of the GHS-R1a receptor (IC50>10,000 nM), since it poorly activated GHS-R1a at micromolar concentrations; large hydrophobic acyl group is obligatory at position 3 of ghrelin for its biological response on GH secretion.
The PCT application, WO 01/87335A2, discloses methods of selectively inhibiting ghrelin actions including those on obesity using growth hormone secretagogue receptor antagonists and ghrelin neutralizing reagents. The ghrelin neutralizing reagents are antibodies, single chain antibodies, antibody fragments, or antibody-based constructs.
Specific binding of acylated ghrelin can be found in many peripheral tissues (Papotti M et al. J Clin Endocrinol Metab 2000; 85(10):3803-3807). In these tissues, no mRNA expression of the GHS-R1a receptor could be found, indicating that other receptor (sub)types of receptors that can bind GHS would be responsible for this specific binding. These novel receptor/s may mediate ghrelin's peripheral actions which are, as shown in this invention, efficiently antagonized by unacylated ghrelin.
It would be highly desirable to be provided with pharmaceutical compositions of nonacylated ghrelin for glycemic control in certain metabolic diseases.